1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging method suited to an observation of the dynamic state of a moving material in a body such as blood or cerebrospinal fluid (CSF).
2. Description of the Related Art
A method has been known wherein inversion pulses are applied in order to label, in the form of longitudinal magnetization, an observation target located in a position at a time, and an MRI image is taken after a given time, thereby recognizing the distribution of the labeled observation target (e.g., refer to “Considerations of Magnetic Resonance Angiography by Selective Inversion Recovery,” D. G. Nishimura et al., Magnetic Resonance in Medicine, Vol. 7, pages 472-484 (1988)).
In the case where this method is used, in general, a time when a characteristic electrocardiographic waveform such as an R wave emerges is set as a reference so that RF excitation and echo signal collection for imaging are carried out at a time when a certain amount of time has passed from the above-mentioned time. This utilizes the nature of the flow velocity change of, for example, blood or cerebrospinal fluid that often highly correlates with a cardiac phase. Such a synchronization method is employed because artifacts due to pulsation can be reduced, because the capability to visualize, for example, the blood or cerebrospinal fluid and signal strength are stabilized, and because image quality is improved.
Another method has been known that conducts labeling for longitudinal magnetization by a selective excitation method before echo signal collection for imaging, and takes a plurality of images with variations in an inversion time TI ranging from the time of the labeling to the imaging (e.g., refer to Japanese Patent Application KOKAI Publication No. 2001-252263). The plurality of images obtained by this method is sequentially displayed at regular intervals such that it is possible to observe the dynamic state of a moving material in a body such as the blood or cerebrospinal fluid.
FIG. 24 is a diagram showing a pulse sequence of this prior art example. Waveforms shown in FIG. 24 represent, from top to bottom, an electrocardiograph (ECG) as a synchronization waveform, a radio-frequency (RF) pulse applied to an imaging target, a slice direction gradient magnetic field waveform (Gss), a readout direction gradient magnetic field waveform (Gro), a phase encoding direction gradient magnetic field waveform (Gpe), and a deviation (Δf) of a carrier wave from a center frequency during the application of the radio-frequency pulse. A period P1 is a tag (label) sequence part for labeling the blood or cerebrospinal fluid, and a period P2 is a main pulse sequence part for imaging. For periods P1 and P2, a combination of gradient magnetic field strengths and Δf can be independently set at the time of the selective excitation, and the direction and position of an excited surface can be independently set. Although a case of a fast spin echo method is shown in this example, it is possible to use any imaging method such as a coherent gradient echo method (a true SSFP method, true FISP or balanced FFE method). The number of shots necessary to reconstruct one image is collected during the same inversion time TI. The imaging is repeated with variations in this inversion time TI. A plurality of images taken at different inversion times TI are sequentially displayed to enable the observation of the dynamic state of the cerebrospinal fluid. With regard to parts with motion among parts excited by a labeling pulse LP in period P1, portions outside a labeling region show a low signal intensity on the image because of motion corresponding to a flow velocity during the inversion time TI. This permits the observation of the motion of the blood or cerebrospinal fluid. Another example has been shown wherein one more nonselective IR pulse is added before or after an RF pulse in period P1 in terms of time such that the longitudinal magnetization of the labeled part is substantially brought to an initial state, thereby imaging the motion of the blood or cerebrospinal fluid as a high signal intensity.
As measurement means for the motion of the cerebrospinal fluid, there are also known, for example, a method wherein a radioisotope is injected into a spinal cavity and its motion in a body is traced by several hours by means such as a scintillation counter, and a method that uses a contrast media (metrizamide) to perform a measurement in the same manner by X-ray computed tomography (CT). In spite of an advantage of facilitating the observation of the long-time motion called bulk motion of the cerebrospinal fluid, all of these methods entail a high degree of invasiveness of a test subject. Moreover, since the radioisotope or contrast media is injected in these methods, the inner pressure of the cerebrospinal fluid might change, which affects an observation target.
First Problem: if the inversion time TI is changed, a time from a reference time point to imaging (times TDseq1, TDseq2 in FIG. 24) changes as much as the change of the inversion time TI in the case where the method shown in Japanese Patent Application KOKAI Publication No. 2001-252263 is used to observe the dynamic state of the blood or cerebrospinal fluid. Thus, the flow velocity of the blood or cerebrospinal fluid during collection varies every imaging, so that there is a disadvantage that a signal value changes due to effects other than the difference of the inversion time TI or the capability to visualize the blood or cerebrospinal fluid varies every imaging. Especially when, for example, a fast spin echo method is employed that uses a Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence as a main pulse sequence, an image is formed as the composition of a plurality of echo components with different phase variations due to flow Velocities, and this leads to sharp changes in image values corresponding to the flow velocities due to the interferential action of phases, which is a serious problem.
Second Problem: in the case where the imaging target is the cerebrospinal fluid, the correlation with the cardiac phase is not necessarily high, and the pulsation regularity of the cerebrospinal fluid tends to be lower than that of blood. Therefore, in the case of the observation target such as the cerebrospinal fluid, there has been a disadvantage that it is not easy to judge whether a change is caused by individual imaging variations or by the variation of the inversion time TI, even if images with the sequentially changed inversion time TI are compared.
Third Problem: in normal labeling methods, there has been a disadvantage that it is not easy to know the motion of the cerebrospinal fluid or blood in the whole two- or three-dimensional region because the labeling region is limited to a straight form.
Fourth Problem: when an imaging section is set out of a normal positioning image, it is not easy to set a labeling region at a proper position because a portion with a suspected lesion of the blood or cerebrospinal fluid is unclear on the positioning image for labeling, and there has been a disadvantage that it is not easy to visualize the motion of the cerebrospinal fluid or blood.
Fifth Problem: there has been a disadvantage that clinical knowledge of the circulatory pathways of the cerebrospinal fluid or blood is required for a user of an apparatus in order to carry out the imaging using labeling, and properly setting imaging conditions is difficult.